In most cases, the desired etch profile is square shaped. To obtain this perfect profile, etch and passivation have to be carefully balanced and if need reajusted as the aspect ratio of the structure increase. Excessive sidewall passivation leads to tapered profiles which are desired for shallow trench isolation etch. The lack of sidewall passivation and excessive spontaneous chemical etching leads to undercutting. Ion deflection in very narrow spaces can lead to bowing. At the interfaces of layers such as the interface between the gate oxide and the poly-Si layer, notching has to be avoided.